std::experimental::disjunction - cppreference.com (original) (raw)
 |
Merged into ISO C++ The functionality described on this page was merged into the mainline ISO C++ standard as of 2/2016, see std::disjunction (since C++17) |
|---|
Forms the logical disjunction of the type traits B..., effectively performing a logical or on the sequence of traits.
The specialization std::experimental::disjunction<B1, ..., BN> has a public and unambiguous base that is
- if sizeof...(B) == 0, std::false_type; otherwise
- the first type
BiinB1, ..., BNfor which bool(Bi::value) == true, orBNif there is no such type.
The member names of the base class, other than disjunction and operator=, are not hidden and are unambiguously available in disjunction.
Disjunction is short-circuiting: if there is a template type argument Bi with bool(Bi::value) != false, then instantiating disjunction<B1, ..., BN>::value does not require the instantiation of Bj::value for j > i.
Contents
- 1 Template parameters
- 2 Helper variable template
- 3 Possible implementation
- 4 Notes
- 5 Example
- 6 See also
[edit] Template parameters
| B... | - | every template argument Bi for which Bi::value is instantiated must be usable as a base class and define member value that is convertible to bool |
|---|
[edit] Helper variable template
| template< class... B > constexpr bool disjunction_v = disjunction<B...>::value; | | (library fundamentals TS v2) | | ----------------------------------------------------------------------------------- | | ---------------------------- |
[edit] Possible implementation
template<class...> struct disjunction : std::false_type {}; template struct disjunction : B1 {}; template<class B1, class... Bn> struct disjunction<B1, Bn...> : std::conditional_t<bool(B1::value), B1, disjunction<Bn...>> {};
[edit] Notes
A specialization of disjunction does not necessarily inherit from of either std::true_type or std::false_type: it simply inherits from the first B whose ::value, explicitly converted to bool, is true, or from the very last B when all of them convert to false. For example, disjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value is 2.